Improve borrowck_graphviz_* documentation.

In particular, `borrowck_graphviz_preflow` no longer exists.

src/doc/rustc-dev-guide/src/compiler-debugging.md

@@ -301,7 +301,8 @@ Right below you can find elaborate explainers on a selected few.
 
 Some compiler options for debugging specific features yield graphviz graphs -
 e.g. the `#[rustc_mir(borrowck_graphviz_postflow="suffix.dot")]` attribute
-dumps various borrow-checker dataflow graphs.
+on a function dumps various borrow-checker dataflow graphs in conjunction with
+`-Zdump-mir-dataflow`.
 
 These all produce `.dot` files. To view these files, install graphviz (e.g.
 `apt-get install graphviz`) and then run the following commands:

tests/ui/mir-dataflow/README.md

@@ -42,12 +42,3 @@ each generated output path.
    on *entry* to each block, as well as the gen- and kill-sets that
    were so-called "transfer functions" summarizing the effect of each
    basic block.
-
- * (In addition to the `borrowck_graphviz_postflow` attribute-key
-   noted above, there is also `borrowck_graphviz_preflow`; it has the
-   same interface and generates the same set of files, but it renders
-   the dataflow state after building the gen- and kill-sets but
-   *before* running the dataflow analysis itself, so each entry-set is
-   just the initial default state for that dataflow analysis. This is
-   less useful for understanding the error message output in these
-   tests.)